Electromagnetic radiation sensor and its manufacturing method

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Field of Invention]

The present invention relates to an active pixel solid state photosensor and image generator using CMOS technology.

【0002】

[Explanation of related technologies]

Active pixel solid-state sensors and devices for detecting electromagnetic radiation are well known and widely used. When mounted on a camera system, the pixel array acts as a visual or image sensor that produces an electrical signal that corresponds to the detected light level. Examples of such photosensors are disclosed in EP739039 and WO 93/19489. These sensors, which are implemented using CMOS or MOS technology, utilize a collection junction, which is a region of the semiconductor substrate that is designed to collect charges generated by radiation. These collection junctions are either pn junctions or np junctions, respectively, depending on whether the substrate is p-conducting or n-conductive.

【0003】

An active pixel is configured with a circuit system integrated in the pixel to amplify the charge collected on the element or component that is sensitive to light in the pixel. Active pixels may also be equipped with additional electronics for finer functions, such as filtering, high speed operation, or more extreme lighting conditions. Conversely, passive pixels do not have such a circuit system, and therefore they require an amplifier that is sensitive to charge and is connected to the pixel via a conductive wire or line of metallization. However, one major drawback of active pixel CMOS or MOS sensors is that a large portion of the pixel surface is used for electrical circuitry, thus limiting the collection area for each pixel.

【0004】

The charge-sensitive volume of a collection junction is greater than the depletion layer of that junction because all the charge generated by photons from the collection junction within the recombination length can diffuse and be collected in that junction. Based on this mechanism, sensors with small collection junctions have a larger photoelectric volume. For example, a photosensor with an apparent anterior size or photoelectric region with a diameter of about 30 μm can be formed with a junction of 3 μm × 2 μm and a recombination length of 15 μm. However, in the case of active pixels containing other circuitry (eg, detection circuitry), some charge that would otherwise reach the collection junction will instead be the junction or configuration of the additional circuitry. Captured by the element. These charges taken by the additional circuitry of the pixels are therefore lost and do not contribute to the detected signal. This is the main reason for the low fill factor and low sensitivity of active pixel sensors.

【0005】

In this technique, the photodiode dark current (current not caused by detected electromagnetic radiation) is primarily the edge of the photodiode or silicon and SiO.₂It is known that the charge carriers at the interface between them are generated via heat. This dark current can be significantly reduced by a method called "inversion mode" or "all-phase pinning". In this method, Si-SiO₂Inversion of the interface is performed by applying a dopant layer to the surface of the photodiode. This dopant layer is an embedded channel (useful collection junction) and Si-SiO₂Prevent contact with the interface. This method typically reduces the dark current by the magnitude of two orders.

【0006】

An example of an active pixel device prior to this invention is described in US Pat. No. 5,625,210 by Lee et al., "ACTIVE PIXEL SENSOR INTEGRATED WITH A PINNED PHOTODIODE". There is. The patent exemplifies the integration of an n-well CMOS pinned photodiode with a transfer gate into an image sensing element of an active pixel element. As shown in FIG. 1, the p-type substrate 24 forms a pn photodiode together with the n region 22, and that region becomes an optical active element and stores photoelectrons produced by photons colliding with pixels. n well 22 p⁺By "embedding" under the pinning region 20, the collected photoelectrons are confined in the deeper n region. Photodiode junction is then Si-SiO₂Since contact with the interface 30 is prevented, the generation of dark current at the center of generation at the interface is suppressed. The electrostatic position generated by the pinning dopant region 20 also reduces the effect of any oxide layer charge on the junction potential. Moreover, such photodiodes have better ionization emission tolerance.

【0007】

The pinning dopant region 20 of the photodiode also reduces the capacitance of the collection junction, which allows the sensor's kTC noise and the potential for "ghost" images, that is, the bright image of the previous frame remains in the later dark frame. To reduce. This so-called kTC noise is one of the main sources of noise in image-forming sensors, but is typically expressed as the amount of noise charge (the uncertainty of the measured value of the charge generated by light), which is the capacitance of the collection junction. It is proportional to the square root. Therefore, any reduction in capacitance is equal to reduction in kTC noise. The pinning dopant layer 20 reduces the capacitance of the collection junction by raising the minimum value of the electrostatic potential wells in which photoelectrons are confined. If this potential well is shallower than the transfer bias of the transfer gate 28, the photodiode can be completely depleted or reset in a short period of time. Therefore, with a sufficient reduction in sensor capacitance due to the pinning dopant layer 20, all of the photoelectrons are transferred to the detection circuit system n-well 26 by conducting the transfer gate 28, leaving no charge in the potential well. Therefore, it can contribute to the image of the later frame.

【0008】

The MOSFET is formed on the p-type substrate 24 by a transfer gate 28 between the charge collection n-well 22 and the CMOS detection circuit system n-well 26. By applying sufficient voltage to the transfer gate 28, a depletion region is formed between the two n-wells 22 and 26, thus the charge transfer between the pinned photodiode and the floating diffusion CMOS detection circuit system. N channels are given for. The transfer gate 28 is a gate or electrode that controls the conditional transfer of charge from a photodiode (or other structure containing charge, such as a storage gate) to a register. The only function of the transfer gate 28 is as a switch, which, when properly biased, forms a charge transfer channel.

【0009】

[Summary of Invention]

However, we have recognized some of the disadvantages inherent in the pinned photodiode technology described in the prior art. First, a precise manufacturing process is important for the proper operation of the pinned photodiode. Although the pinning dopant region 20 and n-well 22 are self-aligned with each other, the inconsistency between the transfer gate and the lower n-well 22 and 26 significantly impairs device performance. If the transfer gate 28 does not extend to the edges of both n-wells 22 and 26, then there is a p-type barrier to charge transfer, thus significantly reducing charge transfer even when the transfer gate is sufficiently biased. Or can be removed. Second, some of the photoelectrons that would otherwise reach the photodiode's n-well 22 are instead collected by another circuit system (eg, a detection circuit system) near the collection junction. In other words, the effective fill factor of the n-well 22 is significantly limited because the photoelectrons that would otherwise contribute to the signal are captured by the potential wells of the surrounding circuitry. Third, while low capacitance photodiodes have a low kTC noise level and high transfer efficiency that minimizes "ghost" images, they have a small area and therefore a small focusing volume.

【0010】

The present invention is to provide an alternative device that overcomes the disadvantages of the devices described in the prior art and achieves some of their preferred properties. By utilizing a dual purpose electrode (not a transfer gate) that extends beyond the edge of the collection photodiode, the invention overcomes the sensitivity of device performance over small inconsistencies between manufacturing processes. In addition, the collection mode potential of this dual purpose electrode is tuned to achieve increased charge confinement and collection efficiency, an additional p found in the prior art.⁺The need for dopant layers can be reduced or eliminated. Finally, the present invention improves the filler fill factor of a photodiode by shielding the charge carriers formed on the substrate from photons from the potential wells of the surrounding circuit system.

【0011】

One aspect of the invention includes a detector of electromagnetic radiation. The detector has a first conductive type dopant at a first density density and a semiconductor substrate with an insulating layer on its surface. A collection region with a dopant of a second density density is formed in the surface region of the semiconductor substrate in the second conductive type opposite to the first conductive type. A dual purpose electrode is formed on the insulating layer and extends over at least a portion of the surface of the collection area and at least a portion of the substrate. Preferably, this collection area forms a junction with the semiconductor substrate. In one embodiment, the junction formed is a photodiode.

【0012】

In one embodiment, the substrate further has a first conductive barrier region where the density of the dopant is higher than the density of the dopant on the substrate. In other embodiments, this barrier region extends under the dual purpose electrode, at least in part. In a further embodiment, a second conductive type detection region with a third density density dopant is formed on the surface region of the semiconductor substrate, but without a boundary with the collection region and is connected to the readout electronic device.

【0013】

In another embodiment, the surface region of the semiconductor substrate beyond the collection region is a barrier region having a first conductive dopant having a density density greater than the density density of the semiconductor substrate, and the readout electronic device is within the shielding region. Is formed in. Preferably, at least a portion of the charge carriers generated in the semiconductor substrate under this shielding region is collected by the collection region.

【0014】

In yet another embodiment, there is a pinning region within the surface region with a first conductive type dopant having a fourth density density. Preferably, this pinning region is not covered by the dual purpose electrode. In one embodiment, this pinning region aligns with the dual purpose electrode and extends along the collection region.

【0015】

Another aspect of the invention includes a method of manufacturing an electromagnetic radiation detector. In this method, a step of providing a first conductive type dopant at a first density density and an insulating layer on the surface thereof on a semiconductor substrate, and a second conductive type dopant opposite to the first conductive type are used. It includes a step of forming a collection region by introducing it into a surface region of a semiconductor substrate at a density density, and a step of forming a dual purpose electrode extending on the surface of the collection region on an insulating layer.

【0016】

[Detailed Description of Preferred Examples]

The present invention is applicable to any active or passive pixel structure. The present invention will be described with reference to certain examples and drawings, but the invention is not limited thereto, but is limited only by claim.

【0017】

FIG. 2 shows a first embodiment of the electromagnetic radiation detector of the present invention formed on a semiconductor substrate 24 with a first conductive dopant at a first density density. In the preferred embodiment of FIG. 2, the semiconductor substrate 24 is a p-type silicon substrate. Insulation layer 30, for example silicon dioxide SiO₂Is specifically formed on the surface of the substrate. As shown in FIG. 2, the detector is formed in the surface region of the semiconductor substrate 24 and is a collection region with a second conductive type dopant opposite to the first conductive type with a second density density. Has 22. This collection area 22 is shown as an n-well in FIG. 2 and forms a photodiode junction with the semiconductor substrate 24. The detector has a first shielding region 42a and a second shielding region 42b, each of which has a first conductive dopant at a density greater than that of the semiconductor substrate 24. .. As shown in FIG. 2, these shielding regions 42a and 42b are two p-wells. In the second shielding region 42b, the detection region 26 is formed in the surface region of the semiconductor substrate 24 with a second conductive dopant at a third density density. This detection area 26 does not border the collection area 22 and is coupled to the detection circuit system 32. The detection area 26 and the collection area 22 define a barrier area 34 between them. In FIG. 2, this detection area 26 is n⁺A well, the barrier area 34 is part of a shielding area 42b that borders the collection area 22. A dual purpose electrode 40 is formed on the insulating layer 30 and extends over the surface of the collection region 22 and completely across the barrier region 34 to the edge of the detection region 26.

【0018】

Those skilled in the art will recognize that the first conductive type can be either n-type or p-type, and there are various values of the first, second and third concentration densities compatible with the present invention. Recognize that. The shielding regions 42a, 42b, collection region 22 and detection region 26 are all formed using techniques well understood in the art, such as diffusion or injection. Similarly, the dual purpose electrode 40 and the insulating layer 30 are formed in a conventional manner. The dual purpose electrode 40 extends across the surface of the collection region 22 and completely across a portion of the shielding region 42b that extends to the surface of the substrate 24. The dual objective electrode 40 extends across the identified portion of the shielding region 42b to the edge of the detection region 26. The light collection junction of the present invention can be a photodiode, but either becomes essentially from a depletion or inversion layer on the semiconductor substrate (eg, a surface channel CCD) or is (partially) depleted on the semiconductor substrate or surface. It can also be a junction that becomes essentially from an embedded channel (eg, an embedded channel CCD). Such structures are typically used in CCDs, charge injection devices, photogates, or similar structures. These are called "virtual junctions" because they collect charge carriers generated through light while having the same functionality as physical np junctions.

【0019】

The shielding regions 42a and 42b shield the photoelectrons generated in the substrate 24 from the potential of an arbitrary oxide region or detection region 26. The electrostatic barrier formed at the interface between the shielding regions 42a, 42b and the substrate 24 is a junction of additional electronic devices or other structures in which the charges generated under the shielding regions 42a, 42b are active elements. Prevent it from spreading to. Since there is no such electrostatic barrier under the collection area 22, it can eventually collect the charges generated under other electronic components. The collection junction of the preferred embodiment has a fill factor of almost 100%, which means that almost the entire surface of the exposed pixel contributes to the detected signal of that pixel. Thus, photodiodes can have a small junction area and a small capacitance while having a large collection volume.

【0020】

When the substrate 24 is exposed, electrons are present. When the dual objective electrode 40 is at a low voltage, it causes an electrostatic potential that collects the electrons created in the substrate 24 in region 44 of the collection region 22. Confinement minimizes the recombination of the collected charge with the electronic state of the surface of the collection region 22. In addition, this confinement reduces the effect of any oxide layer charge on the junction potential.

【0021】

When the dual objective electrode 40 is biased to a high voltage, the charge collected in the collection region 22 flows into the detection region 26 through the inversion layer formed in the shield region 42b under the dual objective electrode 40. This high bias of the dual purpose electrode 40 also reduces the capacitance of the photodiode during charge transfer to the detection region 26 by forming the potential wells shallower in the photodiode. This capacitance reduction allows the collected charge to be transferred to the detection region 26 faster and more completely. Therefore, the signal from this preferred embodiment has a small kTC noise component, reducing the possibility of a "ghost" image.

【0022】

The structure and functionality of the dual purpose electrode of the preferred embodiment shown in FIG. 2 is different from the transfer gate of the prior art configuration. The dual purpose electrode of the preferred embodiment of FIG. 2 improves photoelectron collection and retention, forms an inversion layer for transferring the collected charge to the detection circuit system, and reduces the capacitance of the photodiode. Facilitates transfer.

【0023】

While full charge transfer from the photodiode collection area 22 in FIG. 2 to the detection area 26 in FIG. 2 is theoretically possible, there are various effects that may actually cause incomplete charge transfer. .. The complete transfer depends not only on the voltage or voltage pulse applied to the gate 40, but also on the voltage present at the receiving node 26 in FIG. Usually the V on the n-doped side of the electrode_thIs lower than the p-doped side. This effect is also the reason why a certain amount of charge is retained in the n region after transfer. The amount of charge held is fairly constant and can be ignored in normal sensor operation. This effect can be further counteracted by increasing the voltage at 26 and shortening the p-doped portion of the electrode: the resulting electric field facilitates electron transfer.

【0024】

In another embodiment of the photodiode of the present invention, the dual purpose electron 40 is shorter, used in connection with the pinning p-type region 50, and the first conductive type dopant is the fourth, as shown in FIG. Accompanied by the surface of the photodiode collection region 22 at the density density of. Those skilled in the art will recognize that there are various values of concentration density of the pinning region 50 compatible with the present invention, and the pinning region 50 uses techniques well understood in the art, such as diffusion or injection. You will admit that it is formed. The dual purpose electrode 40 extends largely across the portion of the collection region 22 that extends to the surface of the substrate 24. The remaining surface of the collection area 22 has a pinning area 50. Confinement of the collected charge is primarily achieved by the dual purpose electrode 40, which, when set to the appropriate bias, confine the collected photoelectrons within a volume under the dual purpose electrode 40. .. Since the potential below the inversion region 50 is often lower than that below the dual objective electrode 40, the collected charges flow into area 44 below the dual objective electrode 40 where they are stored. When the dual objective electrode 40 is pulsed or biased to a high voltage, the charge collected in the collection region 22 is transferred to the detection region 26 via an inversion layer formed in the shielding region 42b under the dual objective electrode 40. Inflow. The dual purpose electrode 40 of the preferred embodiment of FIG. 3 therefore offers the same advantages as the preferred embodiment of FIG.

【0025】

In addition, this pinning region 50 is a surface or Si-SiO₂By shielding the detection region 26 from the electrons generated through heat at the interface, the dark current of the photodiode is effectively reduced and its ionization radiation tolerance is increased. Such an inversion region 50 therefore allows for a photodiode with a larger collection area than is feasible in other embodiments.

【0026】

In another preferred embodiment, the dual purpose electrode is used in association with the p-type region 60, as shown in FIG. In contrast to the embodiment of FIG. 3, in the embodiment of FIG. 4, the p-type region 60 is on the right side of the dual objective electrode 40, on the surface of the photodiode collection region 22, i.e. (single purpose). It is located between the electrode 40 and the detection area 26. As in the embodiment shown in FIG. 3, the collected charge is collected in a potential pocket 44 under the electrode 40, which is electrostatically induced by the high bias of the electrode 40. However, the electrode 40 is not used to form a channel through which the collected charge is transferred to the detection circuit system 32. Such a channel is actually an n-type region below the p-type region 60.

【0027】

In the embodiment shown in FIG. 4, the reduction of the bias of the electrode 40 lowers the potential in the region 44 until it falls below the potential below the p-type region 60. The charge then flows to a higher potential in the detection region 26. There is no barrier region traversed by the collected charge, so there is no need for the dual purpose electrode to form an inversion layer. The electrode 40 acts as both a confinement of the collected charge and a release element that allows the charge to flow into the detection region 26. In this preferred embodiment, special care must be taken to ensure that there are no unintended barrier regions formed by inconsistencies that impede charge transfer to the detection circuit system 32. Region 60 is a barrier and has a fixed potential height. As soon as the potential at 44 drops below it, charge carriers from 44 flow to 26.

【0028】

Although described as above in connection with a particular embodiment of the invention, it should be understood that the description of the examples is exemplary of the invention and is not intended to be limiting. Various modifications and applications will be recalled to those skilled in the art without departing from the spirit and scope of the invention as described in the claims set forth above.

[Simple explanation of drawings]

[Figure 1]

It is a figure of the photodiode of the pinned diode of the prior art assembled on the p-type semiconductor substrate.

[Figure 2]

FIG. 5 is a diagram of an electromagnetic radiation detector according to a first embodiment of the present invention with a dual purpose electrode.

[Fig. 3]

It is a figure of the detector according to the 2nd Example of this invention.

[Fig. 4]

It is a figure of the detector according to the 3rd Example of this invention.

[Explanation of symbols]

24 semiconductor substrate, 22 collection area, 40 dual purpose electrodes